This invention relates in general to applying size to a yarn product, and in particular to improved apparatus and method for high pressure squeezing in the application of size.
Size is a product and is a treatment for reducing the friction and abrasive action of a loom on the warp or longitudinal yarn bundles, in preparing for a subsequent weaving process. The commonplace application of size takes place by passing the warp yarns through a size bath to saturate the yarns with size, and then removing excess size solution by squeezing the warp between a pair of rolls. The moisture remaining in the warp yarns after the squeezing operation must be removed by passing the warp through a dryer, which typically includes a number of heated drying drums around which the warp passes to remove the remaining moisture by evaporation. The maximum speed of the sizing operation, measured in terms of lineal warp distance per unit time, is limited by the need to completely dry the sized warp, which is rewound for subsequent weaving.
The term "% add-on" is used to describe the amount of sizing product applied to the warp yarn, and this term is usually expressed as the product of the "wet pick-up" (the weight of size bath liquid applied to a unit weight of yarn) times the % solids (the proportion of size bath liquid made up of actual sizing products). The need to provide at least a desired minimum % add-on to the warp yarn has, in the past, conflicted with the desire to minimize the water evaporation requirement of the yarn immediately after passing through the squeeze rolls. The water evaporation requirement is the weight of water to be evaporated per unit weight of yarn, and reducing the water evaporation requirement decreases the amount of energy required to remove that water from a unit length of yarn.
The squeeze rolls of conventional sizing machines typically include a top rubber covered squeeze roll, and a lower stainless steel roll. The lower roll is driven to substantially synchronize its surface speed with the forward movement of the warp yarns, and the rubber covered squeeze roll is held against the bottom roll to provide a squeeze force typically on the order of ten to twenty pounds per lineal inch (PLI) in the "nip" or region of roll contact, in the prior art. While increasing the squeeze pressure theoretically would reduce the water evaporation requirement, the % add-on was reduced below a desirable amount in the past.
Sizing products which can be mixed in higher concentrations (% solids) have become available. These more highly concentrated sizing products permit a greater amount of liquid removal by squeezing, which substantially reduces the water evaporation requirement while still retaining the desired % add-on of sizing product applied to the yarn. In order to obtain the reduced water evaporation requirement, it was initially believed that the squeeze pressure simply could be increased to provide the desired increased liquid removal before the drying operation. Such prior-art attempts to increase size squeezing, however, met with several unanticipated problems. The top rubber-covered squeeze roll of conventional sizing apparatus is driven only by the friction of surface contact with the lower roll, and the combined effects of increased squeeze pressures and inherent slipperyness of sizing solutions caused intermittent or erratic slippage of the top squeeze roll relative to the bottom roll. A substantial amount of relative slippage of the rolls cannot be tolerated, because the warps must be processed under very low limits of longitudinal strain. Moreover, the amount of slippage in rotation of the rubber covered top roll, caused by rubber strain under increased squeeze force, varied as a function of the squeeze loading, so that the degree of warp strain varied depending on the particular selected squeeze loading. Initial attempts to overcome the problem of top squeeze roll slippage, brought on by attempts to decrease the water evaporation requirement by increasing the squeeze loading, thus were not successful.
Moreover, the frequent need to separate the top and bottom squeeze rolls for threading a new warp created problems with producing an effective yet inexpensive top roll drive, problems that were compounded by the need to maintain a selected squeeze loading substantially unaffected by changes in tension of the means for driving the movable top roll. Simply providing the top squeeze roll with a gear to mesh with a gear on the driven bottom roll, according to one prior art proposal, would subject the gears to damage each time the rolls were separated to thread a new warp inasmuch as rotation of either roll while disengaged could cause the gears to misregister and become damaged when re-engaged.